As is known, the kidneys of the living human body are important excretory organs whose main functions are to excrete both the biologically useless materials accumulated in the blood and the superfluous water and to remove them in the form of urine. The living kidney is composed of about 1-1.5 million microscopic subunits called nephrons. Each nephron is composed of two parts: the glomerulus and the tubules. The glomerulus is built up of capillary vessels, and blood plasma is filtrated from blood through the porous wall of these vessels. Then the filtrate drains from Bowman's capsule enclosing the glomerulus to the tube system forming the tubules. The blood follows the tubules in the capillary efferent vessels starting in the Bowman's capsule and reabsorbs such an amount of water and solutes from the filtrate passing through the tubules as is necessary to restore the oncotic pressure value satisfying the Gibbs-Donnan equilibrium. The filtrate then transforms further along the tubules into urine as a consequence of this reabsorption process. It drains into the central funnel of the renal pelvis and hence into the ureter and is stored in the urinary bladder. The process carried out in the glomerulus is an ultra filtration essentially during which a great deal of water and in it dissolved solids are filtered from the blood. The blood loses approximately 50% of its aqueous content at this stage. In the tubules, the major part --approximately 80-90%--of this filtrate is reabsorbed into the efferent blood vessels together with the dissolved materials therein necessary for the blood. The remaining liquid containing also all the biologically useless materials in a high concentration is the urine that passes through the ureter to the urinary bladder. Approximately 180 liters per day of blood passes through the kidneys producing only approximately 1.5 liters of urine.
When kidneys cannot operate properly, useless materials accumulate first in the blood, then in the body and can lead to death within 10-12 days. That is why several methods have long been being experimented with for replacing a kidney unable to perform its task.
One of the methods is the implantation of a healthy kidney replacing the ill one. The disadvantage of this method is that the body rejects the foreign organ and expels it. For preventing rejection the donor is preferably a near relative. If the donor is a stranger or if a removed kidney is transplanted from a recently deceased person then anti-rejection drugs are given to the recipient patient. These drugs can not be taken ad infinitum due to their harmful side-effects; therefore a transplated kidney cannot generally be expected to keep functioning for more than five years using this method.
Another method is dialysis performed by external dialyzers. This method basically replaces the filtration process taking place in the glomerulus. On the one hand the filtrate that is generated and contains several kinds of materials necessary for blood is removed together with water and on the other hand the thickened blood is returned into the body. This method is rather expensive, unpleasant and inconvenient for the patient and can be regarded only as a temporary solution. A patient must spend three or four periods of eight to twelve hours each week connected to the dialyzer. The patient will feel unwell both before and after dialysis. Before dialysis the waste products build up in the body, and after dialysis, there is an upset of the balance of the chemical processes of the body by removal of too much of the solutes.
The other phase of the kidney's work taking place in the tubules has been regarded as biochemical process so far. Therefore no attempts have been made for returning the proper part of the water and the solutes of the filtrate.
Efforts have been made to help patients suffering from kidney malfuncion and calling for dialysis by eliminating the immobility of the patients and making the dialyzers portable and attachable to the patient's body under the clothing (see U.S. Pat. No. 3,864,259). However, not even this method is able to eliminate the above-mentioned disadvantages of dialysis and, moreover, the portability outside the body raises new problems during the contact with the environment.
The U.S. Pat. No. 4,354,933 describes a dialyzer filtering the water and the solutes out of the patient's blood in a closed unit and suggests that the closed unit be built in the patient's body. In one of the having three tubes, an arterial blood tube, a situated having three tubes: arterial blood tube, venous blood tube and a so-called urine tube between them. The urine tube is separated from the arterial blood tube and the venous blood tube by an ultra-filtering wall.
The filtrate from the arterial blood tube is led into the patient's urinary bladder and is removed from the body through it. However, this filtrate still contains valuable, metabolically important molecules missing from the patient's bloodstream. These molecules are intended to be replaced so that the blood which is led through the so-called venous blood tubes while flowing back to the patient's bloodstream is led through a bath consisting of a dialysing solution of a given composition in the other chamber of the closed unit, and from this bath the venous blood receives the valuable filtrated materials missing from the body.
The venous blood thus enriched promotes the absorbtion into the tube system from the arterial blood tube to the urine tube, according to the patent specification.
The physical-chemical feasibility of the scheme according to the U.S. Pat. No. 4,354,933 raises serious doubts. The chemical composition of the described dialyzing fluid has an oncotic pressure that enables the dialyzing fluid to extract water and other dissolved materials from the blood but does not enable it to enriching the same. Therefore it is doubtful what the venous blood could receive from the dialyzing bath.
The described process has many other additional deficiencies. The operation of the system does not seem to be resolvable without ensuring separate sucking. I.e., blood must be pumped through. Because the bulk of the patients suffering from kidney malfunction have also hypertiony, the suggested process would further increase the blood pressure.
The most considerable drawback of that closed system is that it cannot actually be separated from the surrounding environment in a sterile way because a tube leads out of the chamber containing the dialyzing fluid to the open air from the patient's body through the patient's skin. Through this tube the dialysing fluid must be changed every now and then. Such outlets represent a permanent danger of infection and threaten a life of the patient who may be also less able to combat infections.
The common feature of all the earlier approaches is that they can replace partly only the glomerular function of the kidney.